Method and apparatus for forming a workpiece

ABSTRACT

Apparatus for forming a workpiece by deposition welding comprises a welding head having at least two wires of weld material extending therethrough. Each wire is formed of weld material such that an electric current is passed therethrough, the wires become molten and form the weld material of the workpiece.

[0001] This invention relates to a method and apparatus for forming aworkpiece. More specifically this invention relates to a method andapparatus for forming a workpiece by a deposition welding process suchas a metal inert gas welding process other (otherwise known as MIG).

[0002] It is known to produce metal workpieces by the deposition of weldmaterial onto a base plate. Processes such as MIG utilise a weld torchpositioned adjacent the plate on which a workpiece is to be produced, anelectrode being provided within the weld torch such that when anelectric current is passed therethrough an arc is formed between theweld torch electrode and the base plate. This arc generates heatallowing the welding process to take place by melting of the electrodeand forming a weld pool in the surface of the workpiece.

[0003] A similar but alternative process in this field is known astungsten inert gas welding (TIG). In this process the electrode isformed from a metal with a high melting point such as tungsten such thatthe electrode does not melt during the welding process. An electricalarc is struck between the tungsten tip and the workpiece by a currentthat is applied to the electrode. This arc generates heat which thenmelts the material of the base plate allowing a separate welding wire tobe fed into the weld pool and thus form a workpiece. In the TIG weldingprocess, the feed wire that is introduced to form the weld is consumableand is delivered automatically by a wire feed unit.

[0004] The MIG welding process is similar to the TIG process in that awelding torch is provided adjacent the base plate upon which theworkpiece is to be formed. However in the MIG process the electrode isconsumable as it acts as both the arc initiator and the weld wire.

[0005] With both processes it is difficult to produce walls ofcomponents which have variable cross-section thicknesses. Also the TIGwelding process requires part of the base plate to be heated to a moltenstate with the arc prior to the introduction of the weld wire. Thisrequires a large current to be used.

[0006] According to the present invention there is provided apparatusfor forming a workpiece comprising a welding head, said welding headadapted to be located, in use, adjacent to a work surface upon which theworkpiece is to be formed, at least two electrodes being partiallylocated within said welding head and extending therefrom, saidelectrodes comprising wires formed from weld material such that when acurrent is passed through said wires an electric arc is formed betweeneach wire and the work surface such that the wires and the work surfacebecome molten, each of said wires being controlled independently.

[0007] Preferably the wires are controlled independently such that thedistance between said wire ends and said work surface may be controlled.

[0008] At least one of said wires may be arranged such that one of itsends extends from the welding head to a predetermined distance from saidwork surface such that said distance is different to the distancebetween said work surface and an adjacent wire end.

[0009] The wires may be angled toward one another at their welding ends.

[0010] The speed that each wire is fed through the welding head may beindependently controllable.

[0011] The wires may be located within a shroud of gas.

[0012] Also according to the present invention there is provided amethod of forming a workpiece comprising the steps of providing at leasttwo wires of weld material within a welding head positioned adjacent awork surface upon which the workpiece is to be formed, passing anelectric current through said welding wires such that each wire forms anelectrode and an electric arc is formed between the electrodes and thework surface, feeding said wires through said welding head such that aworkpiece of weld material is built up on said work surface, andcontrolling each wire independently as it is fed through said weldinghead.

[0013] Preferably that the speed rate at which each wire is fed throughthe welding torch is independently controlled.

[0014] The voltage supplied to each wire may be different.

[0015] The current supplied to each wire may be different.

[0016] The current through at least one wire may be alternating currentand the current passed through another wire may be direct current. Thewire having alternating current passed therethrough may be adjacent awire having direct current passed therethrough. Embodiments of thepresent invention will now be described by way of example only withreference to the accompanying drawings in which:

[0017]FIG. 1 is a schematic view of apparatus for forming a workpieceaccording to an embodiment of the present invention;

[0018]FIG. 2 is a diagrammatic view of a cross-section through a weldingtorch according to an embodiment of the present invention; and

[0019]FIG. 3 is an illustrative view of a component produced by themethod and apparatus according to an embodiment of the presentinvention.

[0020] Apparatus 10 for forming a workpiece comprises a welding torch 12attached to a computer controlled robot arm 14. A work table 16 isprovided with a work surface 18 for receiving weld material from theweld torch 12. The workpiece is formed on the work surface 18. The table16 is mounted on a pedestal 20 and is movable such that the work surface18 lies at an angle of approximately 90° to the welding torch 12.

[0021] A number of metal wires 22,24 extend through the robot arm 14from a supply in the form of a reel (not shown) The wires 22,24 are fedfrom the reel to the welding torch 12 such that the ends of the wires22,24 extend just below the tip of the welding torch 12. The wires 22,24are manufactured from a suitable welding material such as titanium.

[0022] The apparatus 10 also includes a supply of an inert gas, forexample argon or a combination of inert and active gases. The supply ofthe gas is in the form of a cylinder 26 and a gas pipe 28 leads from thecylinder 26 to the welding torch 12 through arm 14. This gas allows thewelding to take place in an inert environment thus avoiding oxidation ofthe workpiece in addition to being ionised to form the arc.

[0023] The table 16 is mounted for partial movement relative to thepedestal 20, as indicated by the arrow A, about a shaft 21. The supportpedestal 20 in turn, is rotatably mounted as indicated by the arrow Bupon a base 23.

[0024] Control means in the form of a computer 32 controls the movementof the table 16 and the robot arm 14 as well as controlling the supplyof argon and feeding of the metal wires 22 and 24.

[0025] In operation, the table 16 and the welding torch 12 are bothconnected to a supply of electricity and argon is fed via the pipe 30from the cylinder 26 to the head of the welding torch 12 to form anargon shroud around the welding torch 12 and which extends to the worksurface 18. The wires 22,24 form electrodes as does the work surface 18of the table 16. The argon gas between the wire electrodes 22,24 isionised to create an electrical arc struck between the tips of the wires22,24 and the work surface 18. The wires 22,24 are consumable as theyact both as the arc initiator and also as the welding material. Once theelectrical arc has been struck it is maintained by supplying a level ofcurrent or voltage depending on the type of weld desired.

[0026] Now referring to FIG. 2 a base portion 36 of the welding torch 12is shown adjacent a workpiece 38. A chromium plated copper or stainlesssteel shroud 40 surrounds all but a small portion of the wires 22,24.This shroud 40 contains argon gas which is directed out of the nozzle 42of the shroud 40 during operation. The wires 22,24 are located withinseparate channels 44,46 of the shroud and extend through copper alloycontact tips 48,50 which guide the wires 22,24 into the correctposition. The wires 22,24 are insulated from one another within thewelding torch 12. The copper alloy contact tips 48,50 also provide theelectric current to the wires and therefore maintain the correct arcwelding conditions or parameters.

[0027]FIG. 3 shows a component formed by the welding process A and athicker portion of the wall of the component where both wires 22,24 wereutilised to provide localised thickening of the wall section is shown byB.

[0028] In use the wire weld material of each wire 22,24 melts under theheat produced by the arc and the position of the welding torch 12relative to the table and work surface 18 is controlled by manipulatingboth the position of the welding torch 12 by moving the arm 14 and theposition of the table 16. This control is effected by the control means32. Thus, by appropriate manipulation of the welding torch 12 relativeto the table 16 a component or workpiece, for example for use in a gasturbine engine, can be built up layer by layer by the deposition of theweld material from the wires 22,24. Such layers 52,54,56 are shown inFIG. 2.

[0029] The wires 22,24 are controlled via the computer controller 32 andthe feed rates and voltages supplied may be controlled independently ofeach other. It is therefore possible to deposit varying thicknesses atparticular locations during the building process of the workpiece byincreasing the rate of one wire feed with respect to another. It is alsopossible to remove one wire feed during the welding process so that athin wall may be formed.

[0030] The way in which the molten metal is transferred to the weld poolin the surface of the workpiece 38 is known as the transfer mechanism.Each transfer mechanism has different characteristics and thus issuitable for different types of weld, for example within the corner of aworkpiece or to build onto a thin wall of a component.

[0031] There are three types of transfer mechanisms, short circuit(known as DIP), spray (or globular) and synergic. Each is dependent onthe level of voltage or current supplied to the wires 22,24.

[0032] The short circuit transfer mechanism occurs when a lower level ofvoltage or current is supplied when compared to that needed for theother transfer mechanisms. In this transfer mechanism, the wireintermittently touches the weld pool thus providing short circuitingwhich produces increased resistance and hence heat which melts the endof the wires 22,24 and transfers molten metal to the weld pool.

[0033] Spray or globular transfer of molten metal to the weld pooloccurs at a higher voltage than that needed for the short circuittransfer mechanism. In this case the arc comprises a higher arc energywhich causes the wire 22,24 to melt and provide a spray or globulartransfer of molten metal to the weld pool. The higher arc energy iscaused by increased voltage which correspondingly produces a higherresistance and thus a higher arc energy.

[0034] A synergic transfer mechanism is a combination of both spray (orglobular) transfer and short circuit transfer. In this case a voltagerange is used which provides a controlled pulse of molten metaltransferred from the wire 22,24 to the weld pool. With this transfermechanism the wire 22,24 does not touch the surface of the workpiece 38but is sufficiently distanced therefrom to prevent a spray or globulartransfer mechanism being provided.

[0035] Each transfer mechanism is produced within differentpredetermined voltage ranges which also depend on the wire materialcharacteristics and the wire diameter.

[0036] Each transfer mechanism also provides a different type of weldwhich are suitable for different applications. For example, the spray orglobular transfer mechanism may be chosen for a corner weld where alarge amount of energy is needed to transfer the molten metal into thecorner section.

[0037] In general, a short circuit transfer mechanism is provided whenan extremely small gap exists between the end of the wires 22,24 and theworkpiece 38. For a synergic transfer mechanism a gap of approximately 3to 4 mm is usually provided and for a spray or globular transfer a gapof approximately 4 to 8 mm is provided.

[0038] Each wire 22,24 can be provided with a pre-selected voltagechosen to provide a particular transfer mechanism. The provision of twoadjacent wires 22,24 within the weld torch 12 allows a combination oftransfer mechanisms to be employed thus allowing a greater variety ofweld characteristics to be achieved.

[0039] It is also possible to change the transfer mechanism during theweld deposition process, for example a synergic transfer could be usedby one or both wires 22,24 for the main weld deposition, then a lowervoltage to provide a short circuit transfer mechanism for other featuresof the component to be produced.

[0040] Also it is possible to deposit weld material substantially fasterthan known processes, by using a plurality of wires. The wires 22,24 arelocated adjacent each other, and are angled toward one such that anarrower and higher weld bead 58 may be formed.

[0041] It is also envisaged that alternating current may be passedthrough one wire 22 whilst direct current may be passed through theadjacent wire 24. The electromagnetic fields around each wire 22,24attract one another thus producing a narrower weld bead.

I claim:
 1. Apparatus for forming a workpiece comprising a welding head,said welding head adapted to be located, in use, adjacent to a worksurface upon which the workpiece is to be formed, at least twoelectrodes being partially located within said welding head andextending therefrom, said electrodes comprising wires formed from weldmaterial such that when a current is passed through said wires anelectric arc is formed between each wire and the support member suchthat the wires and the substrate become molten, each of said wires beingcontrolled independently.
 2. Apparatus for forming a workpiece asclaimed in claim 1 wherein the distance between said wire ends and saidwork surface may be independently controlled.
 3. Apparatus for forming aworkpiece as claimed in claim 2 wherein at least one of said wires isarranged such that one of its ends extends from the weld head to apredetermined distance from said work surface such that said distance isdifferent to the distance between said work surface and an adjacent wireend.
 4. Apparatus for forming a workpiece as claimed in claim 1 whereinthe wires are angled toward one another at their welding ends. 5.Apparatus for forming a workpiece as claimed in claim 1 wherein thespeed that each wire is fed through the welding head is independentlycontrolled.
 6. Apparatus for forming a workpiece as claimed in claim 1wherein each of said wire is located within a shroud of gas.
 7. A methodof forming a workpiece comprising the steps of: (a) providing at leasttwo wires of weld material within a welding head positioned adjacent awork surface upon which the workpiece is to be formed; (b) passing anelectric current through said welding wires such that each wire forms anelectrode and an electric arc is formed between the electrodes and thework surface; (c) feeding said wires through said welding head such thata workpiece of weld material is built up on said work surface. (d)controlling each wire independently as it is fed through said weldinghead.
 8. A method as claimed in claim 7 wherein the speed rate at whicheach wire is fed through the welding head is independently controlled.9. A method as claimed in claim 7 wherein the voltage supplied to eachwire is different.
 10. A method as claimed in claim 7 wherein thecurrent supplied to each wire is different.
 11. A method according toclaim 7 wherein the current through at least one wire is alternatingcurrent and the current passed through another wire is direct current.12. A method as claimed in claim 11 wherein the wire having alternatingcurrent passed therethrough is adjacent a wire having direct currentpassed therethrough.